That is the most wanted features that prevented my current organization from upgrade to Sitecore 8.0.

Having pretty complex infrastructure, big codebase, number of legacy code, wide range of third-party external services and data providers altogether with lack of human resources, it was not easy task to perform such a massive migration (from 6.6) at one time. The decision was taken to migrate part by part, starting with CMS itself without analytics, leaving xDB to second phase of upgrade.

But it happened that in 8.0 Sitecore had wired dependency on MongoDb and you had to have it regardless whether you plan to use analytics or not, even for content authoring environments. As we are limited in resources, we chose Sitecore xDB Cloud as the solution for our analytics, but it turned out that xDB Cloud was yet in beta, and business took a (conservative, as usual) decision to wait until it goes out of beta.

We consulted with Sitecore - they advised us about that issue would be sorted out in 8.1 - and that is exactly what happened. So, welcome CMS-only mode!

First of all, what is that?

As per documentation, CMS-only mode separates CMS functionality from analytics and drops CMS dependencies from xDB for those who don't need them. That also related to licensing changes in 8.1 - now there should be an explicit license for xDB functions (re-)issued for 8.1 and later, otherwise (and if you got license from any of previous versions) Sitecore will run in CMS-only mode by default.

There is a boolean configuration switch in Sitecore.Xdb.config file to enable / disable CMS-only mode:

<setting name="Xdb.Enabled" value="true" />

To draw a line between CMS and xDB, Sitecore revised all existing functions and introduces 3 modes of compatibility: Fully compatible, partly compatible and incompatible. As per documentation distribution looks like below:

Fully compatible:

Content editing in the Experience Editor

Device detection

IP Geo-location detection

Experience Explorer

Partly compatible

Campaign Creator – runs without analytics functionality

Commerce Connect – currently incompatible, but will run with limited functionality in future releases

Personalization – in-session personalization works, while personalization based on historical data is unavailable

Web Forms For Marketers – will provide a SQL solution in the future

Incompatible:

Content testing

Email Experience Manager

Experience Analytics

Experience Profile

List Manager

Path Analyzer

Segmentation

To my mind, introduction of CMS-mode is big step ahead; also such a separations of functions is logically cleaner than it was before in 8.0. For example previously you could run Sitecore 8.0 with old license and could even see and run, let's say Federated Experience Manager, but unless you had an explicit license for 8.0 you wouldn't be able to see any parent for placeholder. That was confusing and led to misunderstanding in past, but has more sense.

Hope you are going to get use of CMS-only mode and understanding how it works!